Halides dissolving metals

Inorganic Chlorides/Halides — These metallic salts are formed from the reaction of a weak base with the strong acid HCl. Salts such as these dissolve in water to produce a markedly acidic solution. This is exemplified by aluminum chloride, which is corrosive due to the acidity resulting from the hydrolysis that produces aluminum and chlorine ions. Anhydrous AICI3 hydrolyzes violently when contacted by water. 

Ion exchange, in which cation and/or anion resins are used to replace undesirable anionic species in liquid solutions with nonhazardous ions. For example, cation-exchange resins may contain nonhazardous, mobile, positive ions (e g., sodium, hydrogen) which are attached to immobile acid groups (e.g., sulfonic or carboxylic). Similarly, anion-exchange resins may include nonhazardous, mobile, negative ions (e.g., hydroxyl or chloride) attached to immobile basic ions (e.g., amine). These resins can be used to eliminate various species from wastewater, such as dissolved metals, sulfides, cyanides, amines, phenols, and halides. 

Table 13. Electrochemical carboxylation of organic halides at dissolving metal anodes [174]...

Silver and mercuric ions are relatively specific in their action on alkyl halides. The explanation of this fact is, of course, the great affinity of these metal ions for halide ion as evidenced by the small ionization of mercuric halides, the insolubility of silver halides, and the fact that the silver halides dissolve in excess halide ion to form complexes of the type AgX2 (21). 

One of the most interesting aspects of molten salt chemistry is the readiness with which metals dissolve. FOr example, the alkali halides dissolve large amounts of the corresponding alkali metal, and some systems (e.g., cesium in cesium halides] are completely miscible at all temperatures above the melting point. On the other hand, the halides of zinc, lead, and tin dissolve such small amounts of the corresponding free metal that special analytical techniques must be devised in order to estimate the concentration accurately. 

The mechanism of hydrogen evolution has been investigated by impedance measurements [371] and hydrogen-tritium kinetic isotope effects [375]. The effect of halides dissolved in solution has been studied [372, 376] these ions increase the overpotential in the sequence Cl- Adsorption isotherms for halides have been derived. They conform to the Temkin adsorption model with partial charge transfer. The lateral interaction between adsorbed particles has been calculated. It is higher for Br- than for I- and increases with overpotential on account of the weakening in the metal-halide bond. Thus, halides are substantial poisons for hydrogen evolution on iron. Poisons also include metal ions such as Cd2+, Zn2+, and Mn2+ [26]. 

Primary, secondary, and tertiary alkyl halides also can be reduced with dissolving metals. The primary reduction product is an organometallic compound. Whether the latter is formed quantitatively or whether it is converted into the corresponding hydrocarbon by protonation depends on the solvent. The organometallic compound is stable in aprotic solvents (hexane, ether, THF), while it is protonated in protic solvents (HOAc, alcohols). 

In the 1990s John Wilkes and coworkers introduced air- and water-stable ionic liquids (see Chapter 2.2) which have attractive electrochemical windows (up to 3 V vs. NHE) and extremely low vapor pressures. Furthermore, they are free from any aluminum species per se. Nevertheless, it took a while until the first electrodeposition experiments were published. The main reason might have been that purity was a concern in the beginning, making reproducible results a challenge. Water and halide were prominent impurities interfering with the dissolved metal salts and/or the deposits. Today about 300 different ionic liquids with different qualities are commercially available from several companies. Section 4.2 summarizes the state-of-the-art of electrodeposition in air- and water-stable ionic liquids. These liquids are for example well suited to the electrodeposition of reactive elements such as Ge, Si, Ta, Nb, Li and others. 

Carbanions occasionally react with aryl halides spontaneously, mostly under irradiation, or by supplying electrons either from dissolved metals or from a cathode. However, certain Fe+2 salts catalyse the S l reactions with carbanions. That was the case for the reaction of PhBr or Phi with acetone or pinacolone enolate ions in liquid ammonia or DMS0172a, as well as for the reaction of the enolate ion of several carbanions with several aryl and hetaryl halides in DMS0172b. Since these reactions are inhibited byp-DNB andp-cymene, and the relative reactivity of nucleophiles is similar to that determined in photo-stimulated or spontaneous reactions, it seems that FeCl2 initiates the S l process. 

F -centers (i.e., two electrons trapped in the same anion vacancy) and M-centers (two electrons occupying two adjacent anion vacancies, i.e., two adjacent F-centers) can also be present. Symons [78] observed by electron spin resonance spectroscopy that in solid alkali halide crystals doped with metal the F-centers are the most stable, while F -centers and M-centers may be formed at higher concentrations of trapped electrons. Durham and Greenwood [79] proposed that the dissolved metal dissociated into metal cations and nearly free electrons scattered in the conduction band. 

A good example of the carbonylation process is the reaction of the tetracarbonyl ferra dianion [Fe(CO)4 ] with alkyl halides. This reagent is made by dissolving metal reduction of the l electron Fe(0) compound Fe(CO)5. Addition of two electrons would give an unstable 20-electrc species but the loss of one of the ligands with its two electrons restores the stable 18-electron stru ture. 

The dimeric trihalides form discrete molecules (Fig. 6-2). The halides dissolve readily in aromatic solvents such as benzene, in which they are dimeric. As Fig. 6-2 shows, the configuration of halogen atoms about each metal atom is far from ideally tetrahedral. The formation of such dimers is attributable to the tendency of the metal atoms to complete their octets. 

Most ionic halides dissolve in water to give hydrated metal ions and halide ions. However, the lanthanide and actinide elements in the +3 and +4 oxidation states form fluorides insoluble in water. Fluorides of Li, Ca, Sr, and Ba also are sparingly soluble, the lithium compound being precipitated by ammonium fluoride. Lead gives a sparingly soluble salt PbCIF, which can be used for gravimetric determination of F . The chlorides, bromides, and iodides of Ag1, Cu1, Hg1, and Pbn are also quite insoluble. The solubility through a series of mainly ionic halides of a... 

These are listed in Table 17-C-2. The anhydrous Cr11 halides are obtained by action of HF, HC1, HBr, or I2 on the metal at 600 to 700°C or by reduction of the trihalides with H2 at 500 to 600°C. Chromium dichloride is the most common and most important of these halides, dissolving in oxygen-free water to give a blue solution of Cr2+ ion. 

In general, the dissolving metal or electrochemical reductions of acyl halides or acyclic anhydrides are not useful for the preparation of primary alcohols. Such reductions invariably provide acyloin esters, ene-diolate diesters or related species. Cyclic anhydrides may be reduced to give lactones. For example, the reduction of phthalic anhydride at a mercury cathode has been used in the synthesis of phthalide (90%). In general, however, such reduction are not widely employed in synthesis. 

This chapter is devoted to the discussion of the reduction of carbon-carbon double and triple bonds by noncatalytic methods, These methods include reductions by diimide, by dissolving metals in the presence or absence of proton donors, by low-valent metal ions, by metal hydride-metal halide combinations and by so-called ionic hydrogenation procedures. Of these widely diverse methods of reduction of carbon-carbon double and triple bonds, the reduction by diimide appears to be the most versatile. The reduction of carbon-carbon double and triple bonds by diimide occurs with complete stereoselectivity and stereo-specificity, and can be effected in the presence of a variety of other, very chemically reactive functional... 

Hydrogenolysis ofAllyl and Benzyl Halides and Related Compounds 4.7.4 DISSOLVING METAL REDUCTIONS... 

The reduction of alkyl halides by solutions of dissolved metals like, e.g. sodium in ammonia or alkali metal naphthalenides in tetrahydrofuran, provides a convenient means of removing halogens to produce hydrocarbons or to prepare alkali metal organic compounds. It is generally accepted that these reductions involve free radical intermediates R pathway A, Scheme 12) . 

X = halide M = alkali metal SCHEME 12. Dissolving metal reductions... 

Most of the alkali metal and barium halides dissolve in water without changing the pH, although there is undoubtedly some hydration of the ions. The alkali metal fluorides give slightly alkaline reactions, however, because of the comparatively low ionisation of HF ... 

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